1. Technical Field
The present invention relates to a glass base material elongating method and a glass base material elongating apparatus for heating and elongating a glass base material to form a glass rod with a thinner outer diameter.
2. Related Art
When manufacturing a quartz glass rod, which is representative of an optical fiber preform, a large glass base material is manufactured in advance, and is then elongated using an elongation furnace to obtain a thinner glass rod.
In recent years, when manufacturing optical fiber, there has been a trend of using optical fiber preforms with larger diameters in order to improve mass production. Accordingly, the difference in the outer diameter between the glass base material and the elongated glass rod has become smaller. Glass rods with diameters up to approximately 90 mm can be elongated using a glass lathe, and therefore conventional methods that include performing rough elongation using an elongation furnace and then performing precise elongation using a glass lathe have been common. However, for glass rods having diameters of 100 mm to 120 mm or more that are to be used as optical fiber preforms, it is not easy to perform elongation using a glass lathe, and even when it is possible, the cost is excessive. Therefore, there has been increased need for improving the restriction of outer diameter using an elongation furnace, without needing to use a glass lathe.
In order to improve the outer diameter control ability of the elongation process, it is necessary to narrow the region of the glass base material being heated. If portions that experience a large amount of diameter change, such as the border between a trunk portion and a tapered portion of the glass base material, are not heated locally, the elongated glass rod will be greatly affected by the shape of the glass base material prior to the elongation. However, although the diameter control ability can be improved by narrowing the region being heated, the pulling force needed to soften and deform the glass has a strong inversely proportional relationship to the length of the portion to be softened and deformed. Furthermore, when elongation is begun, the glass base material is heated and softened, and the lower portion of the pulling dummy rod is pulled and elongated by the pulling roller. However, due to reasons such as the temperature of the glass not being raised enough when elongation begins and the heated region being narrower than during normal elongation, the pulling force could be less than the force used during normal elongation for the trunk portion of the glass base material, which results in the desired shape not being realized.
When such a large pulling force is needed for the elongation, the contact surfaces between the pulling rollers and the pulling dummy rod can slip, causing the actual pulling speed to be lower than the set pulling speed. As a result, the diameter of the glass rod is greater than the intended elongation diameter. When the heated region is narrowed in order to improve the outer diameter control ability, this slipping between the pulling rollers and the pulling dummy rod is particularly noticeable during the start of the elongation. In order to decrease the pulling force, the temperature during the initial elongation period can be set to be higher than the temperature during normal elongation (normal temperature), or the feeding speed and pulling speed can be can be set to be lower than the speed during normal elongation (normal speed). However, changes in the temperature or feeding speed cause changes in the shape of the softened glass portion and the temperature distribution, which creates problems such as fluctuations in the outer diameter of the elongated glass rod. As a result, the objective of improving the outer diameter control ability using an elongation furnace cannot be realized.
In order to solve these problems, it is an objective of the present invention to provide a glass base material elongating method and a glass base material elongating apparatus that, when elongating a glass base material to obtain a thinner glass rod, can restrict slipping of the pulling rollers on the pulling dummy rod or glass rod and can improve the outer diameter control ability of an elongation furnace.